I own a 20" (standard weight, 1-7" twist) Colt. I'm also an engineer, so I'm going to look at this from a straight spec. viewpoint:
A Bushmaster or Colt "shorty" (16" barrel) is a nice choice, but in this scenario I would go Bushmaster. While Colt's have slightly better accuracy because of their great out-of-the-box barrel, like you stated you want a carbine and will not need much accuracy beyond 300 yards, so any standard barrel will do for you. The Bushmaster is the best option because 1.) it's probably cheaper than the "Colt Competition Carbines", and 2.) Colt uses non-standard pins/parts/auto-sear block (which is now an issue for me, because I'm installing a new trigger, but will not be able to switch and reattach this trigger to any other lower receiver that I have, because the pins will be different).
However, in your case, you just described in almost exact detail the rifle that AR15.com is selling, called the "AR15.com Limited Edition Group Purchase" rifle (or LEGP). It's a...
- 16" non-chrome lined barrel (for improved accuracy)
- Fixed-telescopic 'M4' style buttstock
- Lower receiver with improved handgrip and safety selector (the lower receiver, I have heard, is the most impressive part of this piece...and a good lower should always be one of the top decisions in your rifle purchase)
- Flat-top, picattiny (sp?) rails...and they offer a set of tactical add-ons, which includes an incorporated COMPLETE rear-sight assembly (windage, elevation, and all) and a weaver rail in front, perfect for scope or AimPoint sights, etc.
I am considering buying this rifle simply for the LOWER RECEIVER ALONE, but overall I have heard almost nothing but good things from the guys who have actually gone up to the shows and seen it. Many members here, upwards of 100, have purchased the rifle. If you don't consider a Bushmaster 16" V-match (also a wise buy), then the LEGP is MOST DEFINATELY for you.
Jewbroni~